Machining processes for boring cylinder blocks that make up engine components include a boring process (a fine boring process or the like) for forming a bore to a predetermined diameter inside of a cylinder block, and a grinding process (a honing process or the like) for subsequently grinding the bore surface. If the boring and grinding processes can consecutively be performed on a single main shaft of a machine tool, then the cylinder block becomes free of misalignment with the tools, which otherwise would be caused if the cylinder block were moved between different machine tools and chucked again. Therefore, the cylinder block can be machined properly and efficiently. For consecutively performing a boring process and a grinding process, the machine tool disclosed in Japanese Patent Publication No. 60-052883 includes a mechanism incorporated within a main shaft, for enabling a boring tool and a grinding tool to project alternatively and radially outwardly from the main shaft.
Japanese Patent No. 3270683 discloses a machine tool having a first support unit and a second support unit for supporting a main shaft, in order to dispense with an oscillation mechanism for changing the main shaft actuating processes between a boring process and a grinding process. During the boring process, the first support unit and the second support unit are pressed against each other for increased rigidity. During the grinding process, the second support unit is spaced from the first support unit, thereby facilitating reciprocating movement through weight reduction.
In the machine tool disclosed in Japanese Patent Publication No. 60-052883, a balancing weight having substantially the same weight as a support unit is connected to the support unit by a wire, so as to compensate for the weight of the support unit. However, this results in the machine tool having a large size and an increased weight.
In Japanese Patent No. 3735487, the present applicant has proposed an invention directed to a honing process for rotating and moving back and forth a honing head, which comprises a tool and grinding stones mounted on an outer circumstantial surface of the tool, inside of a cylindrical hole. According to the disclosed invention, if the overrun dimensions of the tool head at one end and other end of the hole are different from each other, then a schedule for making up for the difference between the overrun dimensions of the tool head at the respective ends is inserted into a schedule for moving the tool head back and forth, depending on the difference between the overrun dimensions of the tool head at the respective ends. In accordance with such a schedule that has been compensated for by the overrun dimensions, although the grinding stones overrun in the honing process mechanism, the amount per unit time at which the grinding stones are not performing grinding is large at the portion of the hole in the workpiece between the respective ends thereof is ground, that is, the portion between the respective ends of the hole where an amount per unit time at which the grinding stones perform grinding is small, is ground. According to the honing process, it is desirable to position the tool head with high accuracy.
In the machine tool disclosed in Japanese Patent No. 3270683, during the honing process, the second support unit is reciprocally moved in spaced relation to the first support unit. This acts to reduce the inertial mass of the moving body and to accelerate the moving body quickly under a constant driving force, so that the moving body can reach a desired high speed when it is reciprocally moved.
With the disclosed mechanism, however, although the moving body is lightweight, it tends to flex because rigidity attained by only the first support unit is small. The moving body reaches a resonance point in a small number of reciprocating cycles (Hz) per unit time, and becomes difficult to reciprocate at high speed. During the honing process, although the moving body can quickly be accelerated since it is lightweight, the main shaft is reciprocally moved at a relatively low speed, resulting in a longer machining time. The moving body includes the main shaft, a mechanism by which the main shaft is supported, and a mechanism for rotating the main shaft.
Furthermore, the honing process requires a complex structure for separating the first support unit and the second support unit from each other. Further, the movable part is heavy, making it difficult to rotate the main shaft at high speed. Therefore, since the main shaft is rotated at a relatively low speed during the boring process, the machining time tends to be long.
The machine tool used for the boring process and the grinding process can be used to machine a bore inside of a cylinder block, which forms a workpiece, to a nearly perfect circular finish.
However, even if a bore in a cylinder block of an automobile engine can be machined to a perfect circular finish by itself, the bore will become deformed when a cylinder head and a crankcase are assembled onto the cylinder block, in a subsequent production process. Deformation of the bore tends to increase sliding resistance between the bore and the piston when the engine is in use, with the possible result that the engine may fail to produce a desired output power.
Japanese Patent Publication No. 51-025523 reveals that a dummy head, having a hole greater in diameter than a bore in a cylinder block and similar in rigidity to a cylinder head, is mounted on the cylinder block. Then, after the cylinder block has been placed under the same conditions as when the cylinder head is fastened to the cylinder block, the bore is formed in the cylinder block from the larger-diameter hole. Further, various proposals for machining the bore with greater accuracy have been made with respect to the dummy head, for example as shown in Japanese Laid-Open Patent Publication No. 2000-052228.
Japanese Patent Publication No. 61-057121 discloses a cylinder block, which is bored while a cylinder periphery thereof is being pressed by a presser rather than a dummy head.
According to the above conventional art, each time a cylinder body is bored in a production process, a dummy head or the like needs to be attached and detached. Therefore, productivity is lowered. In addition, it is difficult for the dummy part to reproduce the same conditions as a fully assembled product.
For boring cylinder heads, accordingly, there has been a demand for a highly productive machining process, which is capable of reproducing a state closer to that of an assembled product state. In order to realize such a machining process, there is a need for a machine tool capable of performing a more accurate boring process.
The present applicant has proposed a compound machine tool, which is capable of performing a boring process and a honing process in a composite fashion in Japanese Patent No. 3270683. The compound machine tool dispenses with the oscillation mechanism, which otherwise would be used in the honing process, thus saving space along the production line, simplifying facilities, and lowering the cost required to manufacture machined parts. The compound machine tool can machine workpieces with increased shaft rigidity, while highly accurately performing the boring process under higher machining loads.
With the above machine tool, the boring tool exerts radially expanding forces, and is radially positionally controlled under hydraulic pressure. Consequently, it is difficult to control (i.e., finely adjust) the expanding forces and the expanded position, which poses limitations on efforts to perform much more accurate boring processes.
Japanese Patent Publication No. 60-052883 discloses, in relation to such a machine tool, a honing tool head adapted to be mounted on the main shaft of a honing machine tool in facing relation to a workpiece for honing the workpiece. The honing tool head is formed having rough grinding stones and fine grinding stones.
FIG. 33 of the accompanying drawings shows a side view of such a tool head 500. The tool head 500 has rough grinding stones 502 and fine grinding stones 504, which are alternately inserted into holes 505 that are defined radially at equal intervals in the tool head.
As shown in FIG. 34 of the accompanying drawings, the tool head 500 has three rough grinding stones 502 and three fine grinding stones 504 fixed respectively onto rough grinding stone bases 506 and fine grinding stone bases 508. The rough grinding stones 502 and the fine grinding stones 504 are radially movable, i.e., radially expansible and contractible, in the directions indicated by the arrows B, while being guided by the holes 505 in the same positions axially on the tool head 500.
The tool head 500 also has a rough grinding tapered cone (cone shaft) 510 and a fine grinding tapered cone (cone shaft) 512, which are individually slidably inserted therein for expanding and contracting the rough grinding stone bases 506 and the fine grinding stone bases 508.
As shown in FIGS. 34 and 35 of the accompanying drawings, the rough grinding tapered cone 510 has tapers 510a, 510b, including tapered surfaces against which the rough grinding stone bases 506 are held in slidable abutment. Similarly, as shown in FIGS. 34 and 36 of the accompanying drawings, the fine grinding tapered cone 512 has tapers 512a, 512b, including tapered surfaces against which the fine grinding stone bases 508 are held in slidable abutment.
The rough grinding tapered cone 510 has a distal end portion divided into three arms, with gaps 514 defined therebetween. The tapers 512a, 512b of the fine grinding tapered cone 512 are inserted into the gaps 514.
Operation of the tool head 500 for radially expanding and contracting the rough grinding stones 502 and the fine grinding stones 504 when the tool head 500 performs a honing process shall be described below.
For expanding the rough grinding stones 502, the rough grinding tapered cone 510 is lifted in the direction indicated by the arrow A1 in FIG. 37A of the accompanying drawings. The tapers 510a, 510b press against inner slanted surfaces 506a, 506b of the rough grinding stone bases 506. Therefore, the rough grinding stones 502 are expanded radially in the directions indicated by the arrows B (radially outwardly). For contracting the rough grinding stones 502 from the expanded position, the rough grinding tapered cone 510 is depressed in the direction indicated by the arrow A2, and the rough grinding stones 502 are contracted in directions opposite to the directions indicated by the arrows B (radially inwardly).
Similarly, for expanding the fine grinding stones 504, the fine grinding tapered cone 512 is lifted in the direction indicated by the arrow A1 in FIG. 37A of the accompanying drawings. The tapers 512a, 512b press against inner slanted surfaces 508a, 508b of the fine grinding stone bases 508. Therefore, the fine grinding stones 504 are expanded radially outwardly in the directions indicated by the arrows B. For contracting the fine grinding stones 504 from the expanded position, the fine grinding tapered cone 512 is depressed in the direction indicated by the arrow A2, and the fine grinding stones 504 are contracted in directions opposite to the directions indicated by the arrows B (radially inwardly).
The honing process includes a process in which the rough grinding stones 502 and the fine grinding stones 504 are simply expanded or unexpanded and the rough grinding stones 502 and the fine grinding stones 504 machine an inner surface of the workpiece. Further, the honing process includes a process in which an inside diameter of the workpiece is measured by an air micrometer inside diameter measuring device (not shown), and depending on an elapsed machining time and a change in the inside diameter, forces for lifting the rough grinding tapered cone 510 and the fine grinding tapered cone 512 are adjusted, whereby the workpiece is machined by the rough grinding stones 502 and the fine grinding stones 504 while forces for expanding the rough grinding stones 502 and the fine grinding stones 504 are varied. The latter process makes it possible to machine the workpiece with extremely high accuracy.
With respect to the latter process, however, it has been confirmed that, even if the forces for lifting the rough grinding tapered cone 510 and the fine grinding tapered cone 512 are increased, forces for expanding the rough grinding stones 502 and the fine grinding stones 504 may change only slowly, or may not change much at all in reality, i.e., the rough grinding stones 502 and the fine grinding stones 504 cannot be expanded quickly.
The above problem is caused by the existence of reactive forces F1 (i.e., forces acting in directions opposite to the directions indicated by the arrows B) that act from the workpiece when the rough grinding tapered cone 510 is lifted in the direction indicated by the arrow A1 in FIG. 38 of the accompanying drawings, so as to expand the rough grinding stones 502 radially in the directions indicated by the arrows B. As shown in FIGS. 38 and 39 of the accompanying drawings, when the gaps 514 in the rough grinding tapered cone 510 are reduced radially inwardly under the reactive forces F1, the rough grinding tapered cone 510 flexes, thus tending to produce frictional forces F2 between the rough grinding tapered cone 510 and the fine grinding tapered cone 512 (see FIGS. 37A, 38B).
The frictional forces F2 produced between the fine grinding tapered cone 512 and the rough grinding tapered cone 510 due to flexing of the rough grinding tapered cone 510 result in a difference (transmission loss) between the forces that lift the rough grinding tapered cone 510 and the fine grinding tapered cone 512 and the forces that serve to expand the rough grinding stones 502 and the fine grinding stones 504. This difference is the major factor responsible for the above-mentioned problems.